It is previously known in percussion devices for hydraulic rock drilling machines to provide the cylinder of the percussion device with a brake recess. In certain operational conditions a land portion of the impact piston of the percussion device enters this brake recess for the establishment of a braking chamber. Such operational conditions are when the shank adapter of the percussion device because of reduced resistance has been displaced forward in the impact direction, such that the strike of the impact piston against the shank adapter no longer occurs within the region of a desired impact position.
The aim of arranging a brake recess in the cylinder and the co-operating land portion on the impact piston for then establishment of a braking chamber is to decelerate the impact piston in order to at least limit the intensity of impacts against the shank adapter in too far advanced positions thereof. Such impacts otherwise risk resulting in damaging the drilling machine.
The brake recess, which is filled with hydraulic medium, is subjected to an instantaneous pressure increase when a land portion on the impact piston enters therein, resulting in that hydraulic medium under pressure is transmitted on the one hand over a throttling slot being established between wall portions of the land portion and the braking recess, on the other hand forward from the established braking chamber towards and passed the piston guiding device to a draining chamber which is optionally arranged in front of the piston guiding device.
Further, after contact of the impact piston with the shank adapter, a fast return movement of the impact piston will occur, leading to cavitation in the hydraulic medium in the established braking chamber and all the way in between the impact piston and the piston guiding device. This risks damaging the piston guiding device, which usually is made from a relatively soft bearing material.
In order to reduce these problems it has been suggested to form the land portion with a surrounding flange and to form the braking recess correspondingly larger such that a greater volume of oil is activated during braking. This results in a lower pressure in the established braking chamber and thereby reduced damages to the piston guiding device and the seals. The previously known solution also allows the throttle slit between the land flange and the co-operating wall portion of the braking recess to be formed with somewhat increased area, which in turn reduces cavitation risks in case of a rebounding impact piston.